Currently about 25% of patients on hemodialysis therapy for End Stage Renal Disease (ESRD) receive dialysis through tunneled central venous catheters for dialysis (CVCD). The standard anticoagulant lock for these catheters is heparin, which has no antibacterial properties. The greatest risk for ESRD patients using CVCDs access is catheter related bloodstream infection ; (CRBSI), with frequent progression to sepsis. MethLock [unreadable] is a catheter lock comprised of methylene blue (an antibacterial substance) and sodium citrate (7%, an anticoagulant). In vitro ; studies of MethLock have shown that it is bactericidal for all bacteria, even in the presence of diluted blood or culture medium. This Phase I and Phase II grant will support a multicenter randomized trial of the safety and effectiveness of MethLock versus heparin as a catheter lock in tunneled CVCD. The protocol has been approved by the FDA and an IDE granted for MethLock for the trial. The primary endpoints are CRBSI (concordant bacterial culture drawn from peripheral blood and the catheter lumen) and removal of the catheter for patency failure (after demonstration of at least 25% decrease in flow rate during dialysis versus a baseline treatment). Each patient will be followed for up to 6 months, and the size of the study (400 patients) will provide enough data to demonstrate whether MethLock decreases the incidence CRBSI versus heparin while maintaining patency of catheters equal to heparin. The endpoints of the study are objective and easily definable but have not been validated in a full-scale clinical trial of CVCD or catheter lock. In Phase Iwe will implement the protocol at one dialysis center (a "pilot" center) to determine the practicality of the trial, analyzing: the ease of patient enrollment, speed of identification of adverse events, and the general correlation between our defined endpoints and clinical interventions by staff on theCVCDs. After Phase I we will determine whether the protocol needs to be modified and if so will request protocol modifications from the FDA. In Phase II we will implement the protocol in a number of dialysis centers and complete the study as originally proposed or as modified after Phase I.